Steam turbine power plants are routinely designed with moisture removal apparatus for extraction of water entrained in the steam flowing through the turbine or collecting on various surfaces within the turbine. Such moisture is desirably removed in order to minimize blade erosion caused by hot water droplets impinging in the blades and further to abate diminution of turbine efficiency from water within the steam flow. In most instances, removal of such water is enhanced by bleeding some steam from the turbine to thereby transport the accumulated moisture. Such extracted steam contains a significant amount of heat energy and utilization of the energy in the extracted steam-water mixture in feedwater heaters to raise the temperature of condensate being returned to a boiler for conversion to steam. One example of a system for using the extracted steam is shown and described in U.S. Pat. No. 3,289,408 assigned to the assignee of the present invention.
U.S. patent application Ser. No. 07/609,938 filed Nov. 7, 1990 and assigned to the assignee of the present invention describes certain attributes of steam turbine systems employing moisture separator reheaters. As pointed out in that application, rising fuel costs have led to the use of higher initial operating pressures and temperatures and additional reheat features, including an increase in the number of heaters that are employed in a turbine cycle. The higher pressures and temperatures have led to other design developments including provision for higher outlet water temperatures by utilizing superheat of the steam, and drain cooling sections in the heaters that subcool condensate. In some prior applications of steam-to-steam reheater drains, drain fluid is discharged as a mixture of condensed steam and scavenging steam from a high pressure reheater in a moisture-separator-reheater (hereinafter MSR) to the highest pressure feedwater heater where the fluid is combined with steam from a first turbine extraction point. From the highest pressure feedwater heater, the condensed steam and other drain flows are then discharged or cascaded seriatim to lower and lower pressure feedwater heaters until at some point in the cycle, the flows become part of the main feedwater stream.
As previously disclosed in U.S. Pat. No. 4,825,657 assigned to Westinghouse Electric Corporation, the drains leaving the MSR high pressure reheater are considerably hotter than the feedwater leaving the highest pressure feedwater heater, as much as 55.degree. C. (100.degree. F.) at rated load, and in excess of 140.degree. C. (250.degree. F.) at 25% load. Accordingly, the drains must be throttled down to the feedwater pressure prior to heat exchange. This results in a loss in thermal efficiency.
One suggested method of minimizing this loss is to pump the high pressure reheater drain fluid into the outlet of the highest pressure feedwater heater. Major drawbacks of this method are: a) an additional pump is required; b) the difficulty of avoiding cavitation due either to insufficient net positive suction head in steady state conditions or to flashing during transients; and c) disposal of scavenging steam that is used to enhance the reheater tube bundle reliability.
The above-referenced U.S. Pat. No. 4,825,657 describes a method and apparatus for improving the thermal efficiency of steam-to-steam reheating systems within steam turbine generator systems by allowing the reheater drain fluid to be directly added to the feedwater stream without the need for additional pumping through use of a drain cooler. The high pressure reheater drain fluid passes through the drain cooler in heat exchange relationship with condensate from the discharge of the highest pressure feedwater heater. This avoids the loss of thermal efficiency resulting from throttling of the reheater drain pressure. Heat rate improvement is greater when the system is operated at less than 100% load. The disclosed system is set forth in the context of field retrofit application to single and multi-stage moisture-separator-reheaters. These existing systems include drain receivers with level controls. Fluid from high pressure reheater drains is collected in the drain receivers and then directed to a heat exchanger (drain cooler) in heat exchange relationship with condensate from a high pressure feedwater heater. The use of a drain cooler avoids loss of thermal efficiency from throttling of reheater drain pressure.
Conventional reheater drain systems customarily employ a pressure breakdown section between the MSR reheater drain connection and the feedwater heater receiving the drain fluid, and a level-controlled drain receiver to accept the condensed heating steam. There is a significant reliability problem with drain receivers, which frequently produces internal flooding in the tube bundle from the high pressure MSR. Such flooding has contributed to numerous damaged tube bundles, necessitating reduced load operation at impaired plant efficiency.
Further, because of the decrease in heater pressure at low loads, accompanied in many instances with an increase in reheater supply pressure, the percentage of scavenging steam increases with decreasing load. However, an increase in scavenging steam has been shown to have only a small effect on the heat rate of a cycle employing a drain cooler.
U.S. Pat. No. 4,955,200 issued Sep. 11, 1990 discloses a method and apparatus for improving a steam-to-steam reheat system in a steam turbine employing a drain cooler. The utility of a drain cooler is enhanced by installing a condensate bypass line with a control valve to allow adjustment of the condensing capability of the drain cooler by optimizing the amount of scavenging steam in accordance with load conditions, thereby achieving a heat rate reduction. A steam turbine generator employs a steam-to-steam reheating system which utilizes a small component of scavenging steam to prevent moisture build-up in the bottom most tubes of a reheater bundle. The system has a high pressure moisture-separator-reheater with a reheater drain, and several increasingly high pressure feedwater heaters connected in series to heat feedwater. Each of the feedwater heaters has an inlet and an outlet for feedwater. Heating of feedwater is accomplished in a drain cooler which receives fluid from the reheater drain and passes it in heat exchange relationship with outlet feedwater prior to feeding the reheater drain fluid to the highest pressure feedwater heater. The system controls the amount of scavenging steam and the fluid level at the drain cooler heat exchanger to control the heat capacity of the drain cooler and eliminate the need for a drain receiver level control.
Heretofore, it has been general practice to remove accumulated moisture in a low pressure (LP) turbine immediately before the turbine exhaust. As discussed above, such moisture extraction also necessitates some steam extraction. In this final extraction stage, the steam-water mixture is drained to a condenser where the heat in the steam becomes wasted energy. The steam component of this steam-water mass represents not only most of the volume of the mass but also as much as 95% of the total heat energy in the mass. Therefore, the extracted steam is the primary component of the heat energy wasted during this extraction.
A secondary problem occurs in sizing the passages for extracting the steam-water mass at the LP turbine final stage because of the instability of the steam-water mixture and non-equilibrium effects. Heat loss factors such as those from specific piping shapes and internal contours and other factors such as the entrainment rate in the steam and variations in pressure ratio with load changes cannot be precisely known. Moreover, large differences, as much as 40-60%, exist among results based upon accepted models of turbines. Due to such differences, it is common to oversize the passages thereby extracting more steam than necessary and wasting more energy.
The process of improving efficiency in steam turbines is one of attempting to balance optimal thermodynamic characteristics against practicalities of cost. For example, there is an optimal feedwater temperature before the feedwater is returned to the boiler which is lower than the saturation temperature corresponding to the boiler pressure. However, to reach that saturation temperature, the feedwater would have to be passed in heat exchange relationship with extracted steam from the boiler. Such treatment is inefficient since the extracted steam would not have done any work before extraction. Thus, there is a thermodynamic cycle optimum feedwater temperature which, for cost reasons, is generally not met. However, if steam is extracted in order to remove moisture, the loss of efficiency due to steam extraction is compensated by the gain in efficiency in removing moisture.
At most extraction points, there is a significant amount of heat energy in the extracted steam. This energy is partly recaptured by passing the steam in heat exchange relationship with feedwater. As the extraction points move nearer the turbine exhaust, and particularly nearer the exhaust of an LP turbine, the amount of heat energy decreases. The last stage extraction point is at such pressure and temperature that it is common practice to simply dump the extracted steam-water mixture into the system condenser, thereby giving up any remaining heat energy in the extracted steam. As discussed above, there are numerous factors which cause wide variations in the amount of steam extracted at this last stage. Various solutions to this last stage extraction variation problem have been proposed including changing the size of upstream extraction passages and their associated feedwater heaters. Analysis of this type of approach have shown it to be less efficient. Applicants have analyzed the energy in the last stage extraction and believe that an additional increment of heat energy can be recovered from the extracted steam-water mixture by using the steam for feedwater heating. Furthermore, the inefficiencies inherent in oversizing the extraction passages can be compensated by controlling the characteristics of the heat exchanger without changing the passages. Still further, Applicants have found that contrary to present systems, an increase in the amount of steam extracted in the steam/water mixture results in a net efficiency improvement. More particularly, if the steam/water mixture at a higher temperature moisture extraction point such as that associated with line 21 of FIG. 1 is dumped into a lower pressure drain line, the required throttling of the higher pressure drain line results in a net efficiency decrease with any increase in extracted steam. Thus, it has not been believed beneficial to utilize heat exchangers at the inlet to the last stage of an LP turbine.
Accordingly, LP turbine final stage extraction has disadvantages both in substantial heat energy waste during moisture removal, where extraction steam is drained to the condenser and in inherent design uncertainties in sizing extraction passages.